Taxing Cannabis

Cannabis

Cannabis Plants - Cultivation & Yields

Classification & Overview

Authorities disagree about the number of species of plant which constitute the genus Cannabis[i][ii][iii][iv]. Although many authorities continue to class all varieties of the plant, including Hemp and Marijuana, as Cannabis sativa, it is widely accepted that there are three separate species or sub-species: Cannabis sativa being most widely cultivated in the Western World, was originally grown on an industrial scale for fibre, oil, and animal feedstuffs, is characterised by tall growth with few, widely-spaced, branches; Cannabis indica, originating in south Asia, and also known historically as Indian Hemp, was cultivated for the drug content, with shorter bushy plants giving a much greater yield per unit height; Cannabis ruderalis is a hardier variety grown in the northern Himalayas and southern states of the former Soviet Union, having a more sparse "weedy" growth, and is rarely cultivated for the drug content[v]. Those who argue that all three are one species point to the fact that cross-breeding produces viable and fertile daughter plants.

Cannabis is unusual among plants in that it is dioecious, i.e. any one plant may be either male or female. Most everyday flowering plants are hermaphroditic, i.e. flowers contain both stamens (male) and pistils (female), whereas the cannabis plant normally produces flowers of one sex or the other. Although the main psychoactive compound - D9 tetrahydrocannabinol, or THC - is present in leaf and in male plants and flowers, the concentration is usually greatest in the female flowering tops[vi]. In most cultivations, plants are produced from cuttings taken from known female plants.

Cannabis grown for drug content is normally grown under controlled conditions, where male and female plants are separated, allowing the female flowers to grow without producing seed (sinsemilla), and produce the most potent material. Where cannabis plants are propagated from cuttings it is possible to determine the sex of the plant so that only females are grown. Where plants are grown from seed, 50% of plants should be of either sex, although some seed catalogues[vii] have claimed to guarantee female plants. These are possible due to the tendency of some female plants to produce a few male flowers (hermaphrodites), enabling pollination with female genetic material, although the plants resulting from hermaphroditic seeds may be unpredictable, unhealthy, and inbred (especially where cuttings originate from the same mother plant)[viii].

The whole of the cannabis plant, with the exception of the seeds and mature stalk, are prohibited under the Misuse of Drugs Act 1971. However, it is only the dried tops of female flowers which are of value, being the form in which herbal cannabis is normally sold. This material is clearly distinguishable from cannabis leaf, which is not regarded as of "merchantable quality", and as such should only be allotted a nominal value.

The quality and yield of cannabis is governed by two factors, the genetic makeup of the seedstock, and the environment in which the cannabis is grown.

Genetic Factors:

Cannabis can be grown from a variety of genetic sources. The parent stock of the plant determines factors such as growth rate (size), internodal length (bushiness), flowering time, flower colour and development and THC content[ix].

Hemp seed - sold in pet food shops. This is the bottom end of the quality spectrum, producing very tall leafy plants which physically resemble cannabis but have virtually no THC content (<0.3%) Although seeds are irradiated in order to cause sterility, a percentage will flower and may produce plants of substantial size, with appearance similar to other cannabis plants. The THC content will be very low, normally less than 0.3%. Industrial hemp can be cultivated under licence in the UK.

Seeds from herbal cannabis deals. The cannabis grown will be similar to the parent crop, but mature seeds are themselves an indication of poorer-quality cannabis (i.e. not sinsemilla)

Pedigree Seeds - Many companies and internet sites offer "pedigree" seeds to be bought in order to grow cannabis of high quality. There are a wide numbers of cultivars available, from more traditional ‘Skunk, Haze, Big Bud, Northern Lights’ to newer varieties such as ‘White Widow, Bubblegum, Blue Cheese, Jack Herer, Mr Nice, AK47’ and many more.

Cuttings - As seeds for "pedigree" plants can be expensive, plants are frequently propagated from cuttings, making clones of the "mother" plant identical in genetic makeup guaranteeing a female plant. If flowering is prevented, a single plant could produce an unlimited amount of material if cuttings were continuously taken and grown from "mother, daughter, granddaughter (etc.)" plants. There is a limited trade in cuttings between growers.

Breeding - Growers can produce their own seeds from first generation crops, these are sometimes used for breeding purposes. In such cases male plants would be cultivated in a separate isolated system.

Environmental factors:

These include the temperature, soil composition, supply of nutrients and water, the spectrum of light available for photosynthesis and the length of the daylight period. The potential yield is primarily limited by the available space, and also by overfeeding, overwatering, disease or infestation.

Temperature: Cannabis grows most efficiently at summer temperatures of 20° to 25°C. Higher temperatures are tolerated for short periods, but lower temperatures reduce growth and resin development. Although cannabis grows well outdoors in the UK, autumn temperatures are generally too low for optimum flower development and resin production. Indoors, temperature is commonly regulated using thermostatically controlled fans and heaters.

Atmosphere: Indoors it is normal to find oscillating fans of various sizes to circulate air amongst the plants and to stress the stems leading to stronger growth. Extractor fans are very common, usually with ducting to take the stale air to the exterior of the building, and creating negative pressure drawing fresh air into the system and minimizing escape of odours. Air filters are increasingly common to reduce the characteristic smell of cannabis in the vicinity of grow-rooms Depletion of atmospheric CO2 by photosynthesis of plants grown in confined spaces may be countered by proper ventilation, or by additional CO2 from commercial cylinders, burning fossil fuels (e.g. gas heaters) or occasionally from home-brewing beers, ciders or wines in the same area.

Growth Media: Growers use a variety of growing media from plant pots/buckets and growbags with conventional soil and compost types, to hydroponic systems. In hydroponic systems water and nutrients are controlled via a continuous circulation to plants grown without soil in tanks using inert media such as gels, rockwool, clay granules or other materials which retain water and stimulate root growth. Cannabis grows most effectively with a neutral to slightly alkaline pH.

Lighting: Growers can employ a variety of light sources, from the sun to artificial light. Availability of light is a key determinant of yield, as low light intensities produce spindly growth and inhibit development. Artificial sources include

Fluorescent tubes of the Gro-lux type used in aquaria, these produce the correct spectrum for maximum photosynthesis but are of low intensity. Similar tubes are commonly used for propagation of cuttings, often in purpose-built arrays.

Low-Energy Grow-Lights – these are like the household low-energy bulbs but with higher outputs, drawing between 100 and 250 watts. These are becoming increasingly common.

High pressure sodium (HPS) lights give a more reddish/yellow spectrum and are considered to produce more abundant flower growth. These come in sizes from 250W to 1000W, most commonly 600W.

A growing number of UK retailers are supplying such lighting systems suitable for the small indoor grower and the prices commanded by such lights are falling. There is a limited second-hand market, but industrial lights and fittings often use similar bulbs which may be available at public auctions. HPS bulbs are even found lighting some court buildings, and are increasingly used for street lighting in preference to the low-pressure yellow sodium vapour lights.

Photoperiod: The length of the daylight period controls the flowering of the plant. Long days, of up to 18 hours sunlight, cause the plant to grow in size, or vegetatively. For the plant to flower, the day length needs to be 10 to 12 hours (in actual fact it is the night or dark period which must be 12 to 14 hours), to make the plant 'believe' that winter is approaching[x]. Outdoors in the UK, the day-lengths in autumn shorten more rapidly the further north the site is located, falling from 12-13 hours to below 10 hours daylight within a few weeks. This reduces the duration of the flowering season, and so the proportion of flowering material in outdoor plants (including those greenhouses) will be much lower than in indoor systems with a constant 12-hour cycle.

Separate Rooms: Most indoor growers will have more than one growing room. One room will have a day length of 24 or 18 hours, and often use a blue-white light source for optimum vegetative growth. The other room (or rooms) will have a 12 hour day length with blackout conditions if necessary, and frequently use a redder HPS light to induce maximum flower development. It is also common for cuttings to be propagated separately from the main growth area, under fluorescent or lower powered lights.

Available Space: The available space is the clearest limitation on potential yield of cannabis plants if these are grown indoors. The yield of plants at maturity is determined by the height and lateral growth, both of which may be restricted in an indoor set-up.

Vertical Space – the maximum height a plant can achieve is the distance from the base of the stalk (i.e. top of pot or level of tray top in hydroponic systems) to around 15cm beneath the lighting canopy. In practice, plants are usually flowered around 30-60cm beneath the lighting. Once plants are flowering little increase in height is expected. Shorter plants will normally yield less than taller plants of the same variety grown in the same conditions.

Lateral Space – the taller the plant the greater the scope for side-branching, if the spacing between plants is less than around half the plant height the development of side-branches will be restricted. The degree of branching is also dependent on the internodal length. Clearly, where a large number of plants are packed into a small space the yield from each plant will be substantially reduced. Indeed, the overall yield from 50 plants may not be significantly different from the overall yield from 10 plants grown in the same space.

Attrition & wastage:

In any system, the limiting factor on yield is the number of plants which can be grown in the flowering stage at any one time. Where a grower has a large number of seedlings and/or cuttings it is not reasonable to assume that all of these will be grown to maturity.

Poor stock: A proportion of plants grown from seed will die or be unsuitable, through abnormalities in growth and development.

Failure to root: A proportion of cuttings will fail to "take" i.e. develop roots in order to grow independently. Growers will frequently select the healthiest plants for further growth and development, and discard plants which are weedier or underdeveloped.

Male plants: Where plants have not been sexed, up to 50% of seedlings/cuttings, yielding male flowers, will be discarded at an early stage in order to prevent pollination of female flowers.

Poor conditions: The yield of a crop may be substantially reduced by extremes of temperature and humidity, by over-watering or over-fertilising which can permanently damage the plants..

Disease/Infestation: A proportion of plants may die or become weakened, attacked by insects or plant diseases. Several plant diseases can attack cannabis, which in an indoor garden can rapidly destroy virtually the entire crop. Insect pests such as spider mites and aphids (greenfly) are frequently encountered.

Cannabis Yields – Literature Sources

Studies of cannabis plants grown in the UK under outdoor conditions by the Laboratory of the Government Chemist have shown a range of dry weights (gross) per plant of 10.9 to 59.1g [xi], 16 to 106g [xii] and 8 to 80 g [xiii], with mean weights per plant of 30 to 60g. The variations in growth rates and THC content were ascribed largely to weather conditions. Other sources have stressed the variability of yields and the importance of environmental factors[xiv][xv]. Kaa[xvi] studied plants grown outdoors and in greenhouses in Denmark finding median gross weights of 308g and 584g respectively, with a mean yield of 8.7% flowering tops after drying. GW Pharmaceuticals, who have a Home Office Licence to grow cannabis in the UK, report gross yields of 157g-188g m-2 in greenhouse conditions, 251g-397g m-2 indoors under mercury lights, and 516g-573g m-2 under HPS lighting (at between 10 and 17x plants per square metre[xvii].

For indoor cannabis cultivation, Ed Rosenthal, author of a number of books on cannabis cultivation, in evidence to the U.S. Congressional Sentencing Commission, stated that a mature cannabis plant grown under modern indoor conditions can usually be expected to yield 10 grams of marijuana (i.e. dried flowering tops), and that each "marijuana garden" should be treated on its own merits[xviii]. Knight et al[xix] grew plants hydroponically under optimum conditions with mesh support for branches (‘screen of green’) yielding a mammoth average yield of 687g per plant. In more typical hydroponic growing conditions in the Netherlands Toonen et al[xx] reported an average yield of 33.78g per plant, and Huizer et al[xxi] reported an average 22g per plant.

Forensic Evidence & Databases:

In the UK, the Forensic Science Service and Environmental Scientifics Ltd maintain separate databases of seized cannabis plants, with the FSS currently (2010) reporting an average yield of 40g per plant and Scientifics between 50g and 58g per plant (although their 10% trimmed mean was recently quoted at a more reasonable 46.5g[xxii]. Neither of these make any allowance for height, cultivation method or available space, and their figures are dependent on officers submitting representative plants from cultivation systems, rather than the largest plants present. Previously the FSS were reporting an average 10-15g per plant.

A further firm, LGC Forensics, estimates yields from immature plants on the basis of 14g per foot height. The equivalent IDMU figures would range between 7g per foot for long-internode plants to 18g per foot for short-internode plants, and 12g per foot for medium-internode plants. The LGC estimate thus appears a more reasonable basis for estimation than a crude average which takes no account of the size of plants.

In 1998 FSS scientists Bone & Waldron[xxiii] reported “The amount of usable cannabis recovered per plant varies enormously, depending on the bushiness of the plant. Weights recovered have been found to range from 1 to 355 grams per plant. In the more mature plants, flowering head material may be 50-70 per cent of this weight.”

Yields of Cannabis Plants in Previous IDMU Cases

IDMU staff have been acting as expert witnesses in criminal cases involving cannabis plants since 1991 where standard procedures include examination of seized cannabis plants at police stations or forensic laboratories. Since 1994 routine data collection included measurement of heights, internodal lengths (maximum on main stem), and yields of leaf and flowering tops. To January 2011 the database included 2409x plants of which 1478 were ‘mature’ and 931 were immature.

Mature Plants: The IDMU database includes a total of 1478x mature (i,e, over 50% flowering tops) sample and bulk flowering plants, found a range between 0.5g and 298g flowering tops (average 24.3g) from individual mature plants with a height range of 8cm to 2.1 metres. The average proportion of flowering tops (excluding figures over 95% which represent stripped plants) was 64.2%. Heights and maximum internodal lengths (short 4cm or less, medium 4.1-7.9cm, long 8cm plus) were also recorded, and results are shown in table 1 below:

Table 1 – Mature Cannabis Plant Yields in IDMU Cases (1994-2010)

Height

No of

Yield of Flowering Tops

% Yield

Yield by Internodal Length

Plants

Min

Max

Average

Tops

Short

Medium

Long

<25cm

135

0.5g

14.8g

3.11g

66.5%

3.24g

4.54g

2.20g

26-40cm

207

0.7g

30.1g

11.69g

62.0%

16.24g

8.51g

12.45g

41-60cm

229

2.12g

118g

16.25g

65.8%

31.78g

19.15g

5.25g

61-80cm

303

1.58g

154g

27.32g

69.2%

66.36g

34.50g

10.16g

81-100cm

307

2.80g

126g

32.04g

66.4%

46.34g

47.14g

22.05g

101-120cm

156

2.54g

221g

34.65g

66.7%

53.04g

33.43g

27.39g

121cm +

141

11.04g

298g

41.55g

70.4%

68.00g

37.08g

38.53g

Overall

1478

24.32g

64.2%

595mg

393mg

211mg

Clearly the single most important factor in predicting yield is the height of plants, although the relationship is not linear. For a given height, plants with short internodal lengths tend to produce higher yields (595mg/cm) than those with medium (393mg/cm) or long internodes (211mg/cm), although a heavily-branched plant with long internodes will produce a higher yield than a single-stemmed plant with short-medium internodes.

Immature Plants: A total of 931x immature flowering plants were also recorded (defined as comprising 49% flowering tops or less by weight). Extrapolating the yield of leaf material at seizure by a factor of 1.79 (allowing for average 64.2% tops at maturity) gives good correspondence with actual yield figures from mature plants for the corresponding height ranges (‘estimate’ maturity comparison values in table 2 below are compared with mean and median values from the mature plants of corresponding heights).

Table 2 – Immature Cannabis Plant Yields in IDMU Cases (1994-2010)

Height

No of

Average Yields at Seizure

% Yield

Maturity Prediction

Plants

Leaf

Tops

Total

Tops

Estimate

Actual

<25cm

186

2.54g

1.14g

3.68g

31.0%

4.55g

3.11g

26-40cm

89

13.66g

6.03g

19.69g

30.6%

24.45g

11.69g

41-60cm

218

11.46g

5.82g

17.28g

33.7%

20.51g

16.25g

61-80cm

187

15.43g

4.97g

20.38g

24.4%

27.62g

27.32g

81-100cm

101

23.80g

10.14g

33.94g

29.9%

42.60g

32.04g

101-120cm

113

16.18g

5.16g

21.23g

24.3%

28.96g

34.65g

121cm +

37

38.58g

16.78g

55.36g

30.3%

60.06g

41.55g

Total

931

13.67g

5.56g

19.23g

100.0%

24.5g

24.3g

Hydroponic vs organic systems: Where plants are grown hydroponically (in rockwool or clay granules) rather than organically (in compost/soil media), yields tend to be similar with short-medium height plants but higher for taller plants in the range 80-120cm. Yields from plants grown outdoors or in greenhouses are limited by the rapid decrease in day length during autumn, with the highest recorded proportion of flowering tops in outdoor-grown plants being 44%.

Table 3 – Yield by Height and Growth Method

Method

Hydroponic

Organic

Outdoor

Height

n

Tops

Total

n

Tops

Total

n

Tops

Total

<25cm

90

3.50g

5.30g

43

2.28g

3.27g

2

4.90g

14.00g

25-40cm

190

12.06g

19.55g

17

7.56g

11.27g

2

2.22g

10.10g

40-60cm

123

14.30g

22.13g

102

18.76g

28.14g

4

12.08g

17.70g

60-80cm

166

28.77g

44.45g

135

25.90g

33.90g

2

4.10g

11.70g

80-100cm

87

52.87g

80.81g

218

23.68g

35.03g

15

5.91g

21.73g

100-120cm

59

47.06g

68.92g

93

26.20g

39.34g

14

9.73g

29.70g

>120cm

81

38.05g

52.57g

57

44.67g

63.22g

12

28.24g

85.22g

Total

796

24.62g

37.24g

665

23.73g

34.10g

51

12.44g

37.39g

Calculation of Yields

Mature Plants: Where cannabis plants are mature, potential yields can be calculated from yields of sample plants, where sample plants are representative of the crop as a whole. The yield is calculated as the average yield of flowering tops per plant multiplied by the number of flowering plants in the system.

Immature Flowering Plants: Where an intact cannabis plant is flowering but not fully mature the yield at maturity can be predicted using the following reasonable presumptions.

[xi] Baker P.B. Gough T.A. & Taylor B.J. (1982) The physical and chemical features of Cannabis plants grown in the United Kingdom of Great Britain & Northern Ireland from seeds of known origin. Bulletin on Narcotics 34(1) 27-36.

[xii]. Baker P.B. Gough T.A. & Taylor B.J. (1983) The physical and chemical features of Cannabis plants grown in the United Kingdom ... from seeds of known origin Part II: Second generation studies. Bulletin on Narcotics 35(1) 51-62.

[xiii] Taylor B.J. Neal J.D. & Gough T.A.. (1985) The physical and chemical features of Cannabis plants grown in the United Kingdom ... from seeds of known origin. Part III: Third & fourth generation studies. Bulletin on Narcotics 37(4) 75-81.